Method and measuring device for measuring the spectra in adjacent channels

ABSTRACT

A measuring device ( 1 ), for measuring the spectrum of a measured signal in several neighboring channels ( 15   −9   -15   9 ) of a working channel ( 14 ), includes a selective filter ( 9 ) which damps the working channel ( 14 ) more strongly than the neighboring channels ( 15   −9   -15   9 ). Furthermore an analogue/digital converter ( 5 ) is provided which detects the filtered measured signal from all the channels to be measured ( 15   −9   -15   9   , 14 ) in a parallel and broadband manner and converts the above to a digital signal. An equaliser ( 11 ) in series with the analogue/digital converter ( 5 ), equalises the digital signal with a frequency response which is the reciprocal of the frequency response of the selective filter ( 9 ).

This is a United States national stage application of Internationalapplication No. PCT/EP01/07723, filed Jul. 5, 2001, the benefit of thefiling date of which is hereby claimed under 35 U.S.C. § 120, which inturn claims the benefit of German application No. 100 36 877.8, filedJul. 28, 2000, and German application No. 100 43 894.6 filed Sep. 6,2000, the benefit of the filing dates of which is hereby claimed under35 U.S.C. § 119.

BACKGROUND OF THE INVENTION

The invention concerns a method for measuring the spectrum of ameasurement signal in several neighbouring channels adjacent to aworking channel and a corresponding measurement device.

The standard ETSI Specs GSM 11.21 defines the maximum signal level ofincidental emissions in the neighbouring channels adjacent to a workingchannel. To check whether mobile phone stations or base stations meetthis standard, the measuring method according to the invention and themeasuring device according to the invention are used. The measuringmethod and the device according to the invention are however alsosuitable in principle for measuring incidental emissions in neighbouringchannels of signals, in particular mobile phone signals, to otherstandards in particular the EDGE standard or the UMTS standard.

The specification to ETSI Specs 11.21 requires a check of both thespectrum due to modulation and the spectrum due to switching. The GSMsignal is divided as is known into several frames with a frame durationof approximately 4.7 ms. The said specification requires, formeasurement of the spectrum due to modulation, the measurement of 21channels (the working channel and neighbouring channels above and belowthe working channel) for at least 200 frames which are then averaged. Ifthe measurements for each channel are performed separately, in total21×200 measurements must be performed with a duration of at least 4.7ms, so that the theoretical minimum measurement period is around 20seconds. With automatic measurement of mobile stations or base stationsfor example at the end of production, this measurement period isrelatively long, in particular as also further measurements of otherspecifications must be performed. This measurement duration cantherefore hinder the production process.

In addition measurement of the spectrum due to switching must beperformed. The said specification here requires measurement of 9channels (the working channel and neighbouring channels above and belowthe working channel) for at least 20 frames.

SUMMARY OF THE INVENTION

The invention is therefore based on the task of producing a method formeasuring the spectrum of a measurement signal in several neighbouringchannels adjacent to a working channel and a corresponding measurementdevice in which the total measurement duration is substantially reduced.

The task is solved in relation to the method by the features of claim 1and in relation to the measurement device by the features of claim 7.The sub-claims concern advantageous refinements of the invention.

The invention is firstly based on the knowledge that the measurementtime can be significantly shortened if the individual measurementchannels are measured not in series but in parallel. For this it isnecessary to detect the broadband measurement signal and not restrict itbefore the analog/digital converter to the measurement bandwidth of thechannel concerned (e.g. 30 kHz for the neighbouring channel and 300 kHzfor the working channel) but supply the broadband measurement signal tothe analog/digital converter. It has however been found that paralleldetection of the measurement signal is not possible without a furthermeasure according to the invention, as the signal strength in theneighbouring channels, in particular in the neighbouring channelsrelatively remote from the working channel, is approximately 65 dB lowerthan in the working channel. As the analog/digital converter only has alimited resolution and also due to the higher signal strength at theworking channel must not be over-modulated, the problem arises as howthese high dynamic requirements can be fulfilled. According to theinvention therefore it is proposed to arrange a selective filter beforethe analog/digital converter which attenuates the measurement signal inthe working channel substantially more than in the neighbouringchannels. As a result an over-modulation of the analog/digital converterin the working channel is avoided and the measurement signal reaches theanalog/digital converter in the neighbouring channels with a highersignal level. As a result the signal can be quantized relatively welleven in the neighbouring channels. After the analog/digital converter isconnected an equaliser, the frequency response of which is reciprocal tothe frequency response of the selective filter. The influence of theselective filter on the measurement signal is thus compensated beforeanalysis.

Advantageously the selective filter attenuates the measurement signal inthe neighbouring channels increasingly weakly as the distance from theworking signal increases, so that in each case the signal of the nearestneighbouring channels is attenuated even more than the signal in themore remote neighbouring channels. As a result the modulation of theanalog/digital converter can be further improved.

In measuring the spectrum of the working channel it is advantageous toswitch off the selective filter. As the signal of the working signal isin any case the dominant signal, in measurement of the working channelthe selective filter offers no advantages but has the disadvantage thatbecause of the lack of constancy of the frequency response within the300 kHz wide measurement bandwidth in the working channel, i.e. at thesummit of the filter curve, the measurement signal is falsified.

Connected before the analog/digital converter is preferably a variableamplifier or a variable attenuator, the amplification factor orattenuation factor of which is set so that the modulation range of theanalog/digital converter is utilised at least almost completely withoutover-modulation.

The bandpass filter which reduces the bandwidth to the measurementbandwidth, i.e. for example to 30 kHz for the neighbouring channels and300 kHz for the working channel, is preferably before the analog/digitalconverter. The equaliser and the bandpass filter are preferably housedin a digital signal processor which can also contain further signalprocessing elements.

An embodiment example of the invention is described in more detail belowwith reference to the drawing. The drawing shows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a measurement device according to the state of the art,

FIG. 2 an embodiment example of a measurement device according to theinvention, and

FIG. 3 an example of a spectrum of the measurement signal and thefrequency response of the selective filter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of a measurement device 1 according to the stateof the art.

The high frequency measurement signal RF is lowered in a mixing andintermediate frequency stage 2 to the intermediate frequency IF and thensupplied to a bandpass filter 3. The bandpass filter 3 limits themeasurement bandwidth BW to 30 kHz in the adjacent channels to bemeasured in succession. At the same time the bandpass filter suppressesthe signal level of the working channel. This filtered signal issupplied to a variable amplifier 4, the amplification factor of whichcan be set. After the variable amplifier 4 is an element 5 with anon-linear for example logarithmic curve in order to utilise themodulation range of the subsequent analog/digital converter 6 as evenlyas possible, so that the quantization stages are small for signals oflow level and large for signals of high level. The digital signal at theoutput of the analog/digital converter 6 is supplied to a digital signalprocessor 7.

As already described in this arrangement it is disadvantageous that thespectra must be measured in series in all 21 channels due to modulationmeasurement or in all 9 channels due to switching measurement, whichleads to an undesirably long total measurement time. As however only onemeasurement channel is measured in each case, the amplification factorof the variable amplifier 4 can be adapted so that the analog/digitalconverter 6 in each case is modulated at least almost completely. Thehigh signal level in the working channel is suppressed in measurement ofthe neighbouring channels via the bandpass filter 3 so that noover-modulation of the analog/digital converter 6 occurs. With thisarrangement there is no dynamic problem in the sense that simultaneouslysignals with high and low level must be measured with good resolution ofthe signal with low level. Because of this benefit previously serialmeasurement has been retained.

FIG. 2 shows an embodiment example of the measurement device accordingto the invention where elements corresponding to FIG. 1 havecorresponding reference figures.

By deviation from the measurement device 1 shown in FIG. 1 according tothe state of the art, in the embodiment example of the invention shownin FIG. 2 the intermediate frequency IF of the measurement signal issupplied to a selective filter 9 which is marked in FIG. 2 as theGSM-ACP filter, where ACP stands for adjacent channel power. Theselective filter 9 in the preferred embodiment example shown can bebridged by a switch 10 which for example can be actuated by the digitalsignal processor 7. Connected after the selective filter 9 is theamplifier 4 with adjustable amplification factor, the amplificationfactor of which can also be changed by the digital signal processor 7.After the amplifier 4 comes the analog/digital converter 5 whichconverts the measurement signal into a digital signal which is suppliedto the digital signal processor 7. In the digital signal processor 7 isan equaliser 11. The frequency response of the equaliser 11 is designedso that it is reciprocal to the frequency response of the selectivefilter 9 so that the effect of the selective filter 9 and the equaliser11 are mutually compensated. Furthermore the bandpass filter 3 whichlimits the bandwidth to the channel to be measured is also part of thedigital signal processor. The equaliser 11 and the bandpass filter 3 canwith the usual means of digital filtration be implemented in bothhardware and software. As well as the bandpass filter shown in FIG. 2with a bandwidth BW of 30 kHz, a second bandpass filter 3 can bearranged with a bandwidth of 300 kHz to measure the working signal.

FIG. 3 shows the measurement points prescribed according to the standardETSI Specs GSM 11.21 in the signal spectrum. The channel interval is 200kHz. Whereas in the measurement of the spectrum due to modulation boththe adjacent channels 15 ⁻⁹-15 ₉ and the working channel 14 are measuredwith a bandwidth of 30 kHz, in the measurement of the spectrum due toswitching the measurement in the adjacent channels 15 ⁻⁴-15 ₄ takesplace with a bandwidth of 30 kHz, in the working channel 14 however witha bandwidth of 300 kHz. The signal strength expected at the individualmeasurement points is shown with the bar 12 in the spectrum. The verylarge dynamic range is clear. The signal level to be expected in theremote neighbouring channels 15 ⁻⁹-15 ⁻², 15 ₂-15 ₉ is more than 65 dBsmaller than the signal level in the working channel 14.

If the neighbouring channels 15 ⁻⁹-15 ₉ were measured in parallelwithout the selective filter 9 according to the invention, the problemwould arise that the signal level of the working channel 14 which ismore than 65 dB higher would limit the modulation of the analog/digitalconverter 5. For if the amplification factor of the amplifier 4 were setso that the signal level in the working channel 14 did not quiteover-modulate the analog/digital converter 5, the signal in the remoteneighbouring channels 15 ⁻⁹-15 ⁻², 15 ₂-15 ₉ would be more than 65 dBweaker and thus be not or scarcely resolvable by the analog/digitalconverter 5. This problem does not occur in the measurement processaccording to FIG. 1 in principle because of the bandpass filter 3 whichis connected not after but before, as the bandpass filter 3 suppressesadequately the higher signal level of the working channel 14.

The selective filter 9 according to the invention however weakens themeasurement signal at least in the area of the working channel 14,preferably also in the neighbouring areas, so much that adequatemodulation of the analog/digital converter 5 is possible even for remoteneighbouring channels 15 ⁻⁹, 15 ₉. Here the amplification factor of thevariable amplifier 4 can be increased so far that over-modulation of theanalog/digital converter 5 is avoided, the resolution of theanalog/digital converter 5 is however largely utilised. In theembodiment example shown in FIG. 3 the selective filter 9 attenuates thesignal in the working channel by approximately 15 dB. Thus the signallevel in the remote neighbouring channel 15 can be raised byapproximately 15 dB. The frequency response of the selective filter 9 isshown in FIG. 3 for example by curve 13.

The selective filter 9 according to the invention indeed leads to adistortion of the measurement signal. This distortion can however becompensated by the equaliser 11 provided preferably in the digitalsignal processor 7, as the equaliser 11 has a frequency responsereciprocal to the frequency response of the selective filter 9. Thefrequency response of the equaliser 11 has at the midrange frequency forexample attenuation 0, and either side of the midrange falls to around15 dB. The frequency response of the equaliser is shown in FIG. 3 in adotted line as curve 16.

As already described in the measurement due to switching, measurement ofthe working channel 14 with a bandwidth of 300 kHz is required. As canbe seen from FIG. 3 the frequency response of the selective filter 9from −150 kHz to +150 kHz is not sufficiently constant. In themeasurement of the working channel 14 however the selective filter 9 isnot required as here the measurement signal is the signal with thehighest signal level and thus other signals with higher signal levelneed not be suppressed. In this case it is advantageous to bridge theselective filter 9 with switch 10 and at the same time in the digitalsignal processor 7 disconnect the function of the equaliser 11 to avoidfalsification of the measurement by the selective filter 9.

The invention is not restricted to the embodiment example described andcan be used in a multiplicity of other concrete circuit designs.Furthermore the process according to the invention is not restricted tomeasurement processes according to the GSM standard but can be appliedin the same way to other signals in particular other mobile phonesignals.

What is claimed is:
 1. A method for measuring the spectrum of ameasurement signal in several neighbouring channels (15 ⁻⁹-15 ₉)adjacent to a working channel (14) with the following process steps:parallel broadband detection of a measurement signal over allmeasurement channels to be measured (15 ⁻⁹-15 ₉, 14), filtering of thebroadband detected measurement signal with a selective filter (9) whichattenuates the working channel (14) more than the adjacent channels (15⁻⁹-15 ₉), conversion of the filtered analog measurement signal into adigital signal, and equalisation of the digital signal with an equaliser(11), the frequency response of which is reciprocal to the frequencyresponse of the selective filter (9).
 2. A method according to claim 1,characterized in that the selective filter (9) attenuates themeasurement signal in the neighbouring channels (15 ⁻⁹-15 ₉) more weaklyas the distance from the working channel (14) increases.
 3. A methodaccording to claim 1 or 2, characterized in that at the same time thespectrum in the working channel (14) is measured.
 4. A method accordingto claim 3, characterized in that the digital signal in the workingchannel (14) is subjected to a broader band bandpass filtration than inthe neighbouring channels (15 ⁻⁹-15 ₉).
 5. A method according to claim 3or 4, characterized in that in the measurement of the spectrum of theworking channel (14) the selective filter (9) is disconnected.
 6. Amethod according to any of claims 1 to 5, characterized in that themeasurement signal is a mobile phone signal, in particular a signalaccording to the GSM, EDGE or UTMS standard.
 7. A measurement device (1)for measuring the spectrum of a measurement signal in severalneighbouring channels (15 ⁻⁹-15 ₉) adjacent to a working channel (14)with a selective filter (9) which attenuates the working channel (14)more than the neighbouring channels (15 ⁻⁹-15 ₉), an analog/digitalconverter (5) which detects the filtered measurement signal in parallelbroadband over all channels to be measured (15 ⁻⁹-15 ₉) and convertsthis into a digital signal, and an equaliser (11) which equalises thedigital signal with a frequency response reciprocal to the frequencyresponse of the selective filter (9).
 8. A measurement device accordingto claim 7, characterized in that the selective filter (9) attenuatesthe measurement signal of the adjacent channels (15 ⁻⁹-15 ₉)increasingly more weakly as the distance from the working channel (14)increases.
 9. A measurement device according to claim 7 or 8,characterized in that connected before the analog/digital converter (5)is a variable amplifier (4) or a variable attenuator, the amplificationfactor or attenuation factor of which is set so that the modulationrange of the analog/digital converter (5) is fully utilised at leastalmost completely without over-modulation.
 10. A measurement deviceaccording to any of claims 7 to 9, characterized in that a digitalbandpass filter (3) is connected after the analog/digital converter (5).11. A measurement device according to claim 10, characterized in that aswell as the spectra in the neighbouring channels (15 ⁻⁹-15 ₉), thespectrum in the working channel (14) is also measured and the bandpassfilter (3) for the working channel (14) has a greater bandwidth than forthe adjacent channels (15 ⁻⁹-15 ₉).
 12. A measurement device accordingto claim 10 or 11, characterized in that the equaliser (11) and thebandpass filter (3) are part of a digital signal processor (7).